The present invention relates to an improved apparatus and process for crystal growth, and crystals obtained with the apparatus or process. More particularly the invention relates to an apparatus and method for vapour phase crystal growth with non-intrusive growth monitoring, crystals grown with the apparatus or process, in particular for use as a semi-conductor or optical crystals, and the use of known and novel monitoring equipment to monitor crystal growth with the apparatus or process of the invention.
In designing an effective vapour growth system which has the potential for commercial development and the production of large, high quality single crystals of semiconducting materials with for example cadmium telluride (CdTe), there are three major concerns:
1. the achievement of an adequate growth rate. Without being too specific, a rate less than 1 mm per day is becoming unacceptable. The growth rate should also be controllable as it has an important influence on crystal quality and too high a growth rate results in poor quality crystals;
2. the need to achieve high quality single crystal over a 50 mm diameter boule; and
3. the requirement for a user friendly, robust, manufacturable but flexible design.
Until fairly recently conventional vapour transport has involved the use of a simple linear system with a source and sink of single crystals of II-VI compounds, such as CdS, ZnS, which sublime easily from the solid phase. These together with a seed crystal are located in a sealed quartz ampoule in a tubular furnace in an arrangement for example as described in W. W. Piper and S. J. Polich, J. Appl. Phys. 32 (1961) 1278 (see FIG. 1 below). The source and sink are at different temperatures and therefore have different equilibrium vapour pressures. This vapour pressure difference provides the driving force for growth.
This approach results in certain fundamental problem for growth of crystals such as CdTe:
The equilibrium vapour composition of CdTe is non-stoichiometric except at one temperature, the congruent evaporation temperature which is described in more detail in D. de Nobel Philips Res. Repts. 14 (1959) 361. Due to the law of mass action:
xe2x80x83[Cd]Te2]xc2xd=K(T)xe2x80x83xe2x80x83(1)
where [Cd] and [Te2] are the concentrations of cadmium and tellurium vapour respectively and K is a constant depending on temperature, T. N. Yellin and S. Szapiro, J. Crystal Growth 69 (1984) 555 have reported that minute deviations from stoichiometry in the bulk source material result in large variations in the composition of the vapour making the transport and hence growth highly non-reproducible. Furthermore, this effect gives rise to non-stoichiometry in the growing crystal which has a detrimental effect on its useful properties.
Attempting to overcome these problems with the use of high source/sink temperatures is very difficult and does not lead to a significant improvement in growth rate. Alternatively, control of the axial temperature gradient is also difficult in simple tubular systems and it is difficult to thermally isolate source and sink regions as radiation is an important thermal flux. Furthermore, exact determination of the parameters controlling growth (i.e. surface temperatures of source and seed, vapour pressures) is difficult.
This approach may be improved by the use of a reservoir containing one of the constituent elements to control the partial pressures according to equation (1). A limitation with this approach in a typical growth system is that the exact conditions of temperature and partial pressure are not determined directly and so the optimum reservoir temperature may be uncertain requiring analysis of grown crystals. This problem is compounded, in a system without in-situ monitoring, by any change in conditions during a growth run and run to run variations.
Another major advance in overcoming the limitations of this technology was proposed by the NASA/University of Alabama group of Rosenberger, Banish and Duval (RBD) in F. Rosenberger, M. Banish and W. M. B. Duval, NASA Technical Memorandum103786. Their design was a tubular system with a flow restrictor between source and seed continuously pumped via a second flow restrictor immediately downstream of the seed crystal (FIG. 2.). The continuous pumping in conjunction with a suitable sizing of the downstream flow restrictor removed a small proportion of the source material and in particular any excess component preferentially thus maintaining the vapour phase nearer stoichiometry. The first flow restrictor acted to make the mass transport rate relatively insensitive to the temperatures of the source and sink and their difference. If not restricted in this way, in a system operating under near stoichiometric conditions, appropriate transport rates would require the temperature difference between source and sink to be controlled to within a small fraction of a degree which is difficult especially if the temperatures of the source and growing surfaces cannot be measured directly. This system does, however, suffer from some significant limitations:
1. Thermal coupling along the axis of the furnace prevented the desired axial temperature profile from being obtained.
2. Direct determination of the surface temperatures of source and seed was not possible, and indirect determination uncertain due to the complexity of the radiation field.
3. The partial pressures of source species over source and seed were not directly measurable and uncertainties in the flow modelling of the system and its restrictors made indirect determination uncertain.
4. The quartz ware was complex, not easy to use and vulnerable in application.
In-situ optical monitoring is known and routinely employed in other methods for crystal growth, such as low temperature and thin film growth, where the xe2x80x98efficiencyxe2x80x99 of the process is not very important. Examples of this are Molecular Beam Epitaxy (MBE) (see FIG. 3) and Metal-Organic Vapour Phase Epitaxy (MOVPE) (see FIG. 4) however these techniques are not suitable for xe2x80x98bulkxe2x80x99 crystal growth which requires enclosed transport passages for efficient source utilisation and also requires heating of the quartz passages to allow optical access while preventing condensation prior to the growth region.
Accordingly there is a need for an effective vapour growth system which allows the production of large, high quality single crystals as semi-conducting materials with effective temperature and stoichiometry control.
We have now surprisingly found that an apparatus and method for vapour phase crystal growth may be provided which enables in-situ monitoring in non-intrusive manner and moreover allows for substantial thermal isolation of source and sink regions.
In its broadest aspect there is provided according to the present invention an apparatus for bulk vapour phase crystal growth comprising:
at least one source zone and at least one sink zone each associated with means for independent temperature control within the zone; and
at least one passage means adapted for transport of vapour from source to sink zone; and
additionally comprising means for in-situ monitoring of the sink zone;
wherein means for monitoring is substantially non-intrusive in terms of temperature regulation within the sink zone.
Means for independent temperature control are for establishing a temperature differential to enable solid-vapour-solid phase transition in the respective source, transport and sink zones. Temperature control may therefore be selected according to the phase transitions for any given crystal which it is desired to grow, for example in the range from xe2x88x92150xc2x0 C. to +2000xc2x0 C., employing in each case a greater source than sink temperature with use of appropriate cooling and/or heating control.
Preferably means for in-situ monitoring of crystal growth comprises means providing visual and/or radiation access to the growth zone but located remote therefrom. More preferably means for direct monitoring of crystal growth comprises at least one passage for monitoring communication between the remote visual/radiation access means and the sink zone, wherein the at least one passage for monitoring communication and the at least one passage for transport of vapour associated with any given sink zone are coincident for at least that portion of their length proximal to the sink zone.
It is a particular advantage of the invention that the apparatus as hereinbefore defined may be operated with use of conventional or modified visual/radiation monitoring means, located external to the passages as hereinbefore defined, by means of the visual/radiation access means, for example x-ray and the like may be employed to monitor crystal growth. Moreover the apparatus of the invention may be employed in any bulk vapour transport technique with associated advantages in crystal quality, thereby overcoming disruption of growth conditions which are inherent with known in-situ monitoring means proximal to the sink zone.
Reference herein to locations remote from the at least one sink zone is to locations at which the presence of access means as hereinbefore defined introducing temperature variation or gradient in the vapour transport passage would substantially not disrupt the conditions of temperature required for uniform growth, having regard to conditions of temperature created by means of temperature controlling means for the at least one sink zone. In contrast reference herein to locations proximal to the at least one sink zone are to locations which would be subject to substantial disruption of conditions of temperature under these circumstances.
In a preferred embodiment the present invention provides an apparatus as hereinbefore defined wherein at least one of the passage for visual/radial communication and the passage for vapour transport associated with any given sink zone deviates by an angle of at least 5xc2x0 at any one or more points along the length thereof remote from the growth zone, preferably 5xc2x0-270xc2x0, more preferably 30xc2x0-180xc2x0, most preferably 45xc2x0-110xc2x0, for example 60xc2x0-95xc2x0.
Accordingly the passage for vapour transport may deviate by an angle as hereinbefore defined whereby means for visual/radiation access may be located in the wall of the passage for vapour transport in direct line communication with the sink zone. For example means for visual/radiation access may comprise a visual/radiation-transparent port sealed into and optionally continuous with the wall of the transport passage, located opposing to the sink surface.
Alternatively the configuration of respective passages for visual/radiation access and vapour transport as hereinbefore defined may be reversed, whereby the passage for visual/radiation monitoring may deviate by an angle as hereinbefore defined from a direct line communication of source and sink zone. In this case means for visual/radiation reflection is suitably provided in association with the passage for visual/radiation monitoring at its point of deviation, whereby virtual or reflected direct line access is provided with the sink zone. For example a reflective or transmissive means such as mirrored or prism quartz may be provided in association with the visual/radiation monitoring passage at its point of deviation.
It will be appreciated that the present invention in its preferred embodiment as hereinbefore defined provides a simple and efficient means to achieve the desired vapour phase crystal growth overcoming the hereinbefore defined problems in admirable manner. It is a particular feature of the invention that the provision of passages for visual/radiation access and vapour transport which are substantially coincident proximal to any given sink zone enables non-intrusive monitoring as hereinbefore defined.
Preferably the apparatus as hereinbefore defined comprises at least one passage for vapour transport as hereinbefore defined, which deviates by an angle of at least 5xc2x0 as hereinbefore defined along the length thereof between source and sink zones, more preferably deviates by at least 5xc2x0 at two points along the length thereof whereby both zones are adapted to comprise source and sink material free from constraints of gravity, i.e. which are substantially provided on suitable support means and with passage means extending substantially upwardly therefrom, thereby providing for optimal transport with minimum disruption of the growth process.
The apparatus as hereinbefore defined may comprise a plurality of source zones, for example each associated with a passage for vapour transport, which passages may converge or otherwise, thereby having a common or separate passageways proximal to a single sink zone. By this means, a plurality of source zones may be located radially about a common sink zone, or extending outwardly to one side thereof or may be located in suitable pressure or temperature communication with separate sink zones. A plurality of source zones may be independently activated by means of independent temperature control means associated with each source zone, whereby vapour may be generated sequentially or simultaneously from respective source zones. In a preferred embodiment a plurality of source zones may be adapted to contain a combination of different elemental or compound source material providing each element or compound respectively of a binary, ternary or other multinary compound, connected to a single or plural growth zones via flow restrictors. Similarly a plurality of sink zones may be provided adapted to contain a plurality of seed crystals which may be the same or different, each associated with a passageway for visual/radiation and transport proximal thereto, each sink zone having dedicated means for visual/radiation access.
The apparatus of the invention may additionally comprise means for visual/radiation access to the one or more source zones, provided in similar remote manner as hereinbefore defined for access means to the sink zone, preferably such that a passage for visual/radiation access to, and passage for vapour transport from, any one source zone are common proximal to the source zone.
It is a particular advantage of the apparatus of the present invention that both objects of accurate temperature control of source and sink zones and non-intrusive monitoring of at least the sink zones can be achieved in mutually beneficial manner, whereby positioning of monitoring access means between dedicated temperature control means prevents disruption proximal to either zone.
It is a further advantage of the invention that the apparatus is ideally suited to inclusion of a flow restrictor, for example as proposed by NASA/University of Alabama RBD group above, located remote from both zones, for example upstream of sink monitoring means, for the purpose of vapour pressure control. Preferably in-situ means for monitoring vapour pressure is provided associated with a flow restrictor, in the form of known vapour pressure monitoring apparatus, for example as described in J. Carles, J. T. Mullins and A. W. Brinkman, J. Crystal Growth, 174 (1997) 740, the contents of which are incorporated herein by reference.
Flow restrictors may be selected from any known restrictors and preferably comprises a capillary, porous sintered disc or the like.
The apparatus of the invention is suitably constructed of any material which is adapted for use at the temperatures envisaged for crystal growth, for example is constructed of low, ambient or high temperature resistant material. Suitable materials are known in the art and preference is given to metal oxides, and in particular quartz, refractory oxides and graphite having the required mechanical strength and integrity, for example being reinforced with a suitable material providing mechanical strength and integrity. These materials are also preferred for reason of their high purity with low risk of contamination of crystal. Preferably the apparatus comprises a sealed or sealable structure or envelope including zones and passages as hereinbefore defined. The apparatus is suitably operated at reduced pressure and is encased in a vacuum jacket or the like.
The apparatus of the invention may be used for any bulk vapour transport techniques as hereinbefore defined. It is a particular advantage that the apparatus is adapted for growth of crystals from polycrystalline binary, ternary or other multinary compounds. It is a further advantage that the apparatus of the invention is suited for use with growth of crystal from binary, ternary or other multinary compounds requiring stoichiometry control to compensate for a degree of non-stoichiometry in vapour composition of the desired crystal.
The source and sink zones are adapted to comprise source material and seed crystal as known in the art, for example in the form of one or more reservoirs of source material and a crystal of seed material. Preferably the one or more reservoirs of source material comprise material in solid phase supported on a glass or other suitable surface or pedestal adapted to the processing conditions to be employed, allowing convenient and efficient vaporisation.
In a further aspect of the invention there is provided a process for bulk vapour phase crystal growth comprising:
providing at least one reservoir of source material and at least one sink or seed crystal,
each associated with independent temperature control means; and
transporting vapour phase material between source and sink or seed; and
providing in-situ means for monitoring growth of crystal at each sink or seed;
wherein means for monitoring is substantially non-intrusive in terms of temperature conditions at the sink or seed.
Preferably means for monitoring radiation and transport for any given sink or seed is by coincident monitoring and transport path for at least the portion of the respective paths proximal to the sink or seed, as hereinbefore defined.
Preferably the process is operated at reduced, ambient or elevated temperature as hereinbefore defined. The process is moreover operated at reduced pressure, for example in the range up to 1 bar, preferably 0.01 to 100 mbar reduced pressure.
The process may be started up by known means to establish a sufficient vapour pressure above source material to initiate growth.
In a further aspect of the invention there is therefore provided method for starting up the process as hereinbefore defined in manner to establish a sufficient vapour pressure above the source material to initiate transport.
In a further aspect of the invention there is therefore provided a method for starting up the process as hereinbefore defined in manner to establish transport control and temperature control in the sink zone for controlled growth at the sink or seed.
The method for starting up is suitably operated with temperature and transport rate ramping profiles. It is a particular advantage that independent temperature control means provided with the apparatus of the invention enables temperature ramping specific to growth at the sink or seed, which may also be at a temperature lower than that at the source. It is thought that this gives rise to excellent crystal quality and may even prevent an amount of precipitation or eliminate precipitation entirely.
The process is suitably operated with means for in-situ monitoring as hereinbefore defined according to known techniques. Preferably temperature is monitored by known means at the surface of the sink, and optionally of the source, in manner to enable adjustment as required for optimum temperature control and stoichiometry. Likewise vapour pressure is suitably monitored between zones, for example at the location of a flow restrictor and may be adapted or adjusted as required for optimum growth.
Preferably the process of the invention as hereinbefore defined additionally comprises direct reading of process variables, comparison with optimum values of process variables for a desired crystal growth, for example, with use of a process model, and on line optimisation thereof.
The apparatus and process of the invention as hereinbefore described are adapted for growth of any crystal employing bulk vapour transport techniques.
In a further aspect of the invention is therefore provided a crystal grown with the apparatus or process of the invention. The invention is suited for growth of crystals comprising any compounds which are capable of being sublimed, having a significant vapour pressure below their melting point. Preferably crystals are selected from compounds of groups IIA, IIB, III, V, VI and VII, or Group IV, more preferably of group II and V or Group IV of the Periodic Table of the Elements, for example selected from Be, Mg, Zn, Cd, Hg, S, Se, Te and I or from Si and C. Particularly useful crystals grown with the apparatus and process of the invention cadmium telluride and silicon carbide.
In a further aspect of the invention there is provided the use of known monitoring equipment to monitor crystal growth with the apparatus and process of the invention.
In a further aspect of the invention there is provided the use of the apparatus or process of the invention for any vapour transport technique, such as semiconductor or optical crystal growth.